The problem of launching payloads into orbit from the earth's surface involves the development of a thrust generating mechanism powerful enough to lift a payload off the earth's surface and accelerate it to orbital velocities. In the prior art, this has been achieved with the development of high thrust rocket engines using the reaction principle. Unfortunately, rocket engines consume huge quantities of propellant that must be stored onboard the vehicle. Hence, in order to launch even relatively small payloads into orbit, the size of a rocket propelled launch vehicle must be enormous in order to contain all of the propellant. (The required propellant mass is over 100 times greater than the payload mass.)
The "scramjet" designed for the proposed ground-to-orbit aerospace plane, represents another type of propulsion system for accelerating payloads to high velocities. Basically, this engine is a ramjet in that it ingests air at some ingestion velocity (which is essentially equal to the engine's forward velocity through the atmosphere), sprays it with fuel which ignites forcing the expanding combustion gases to leave the engine with a velocity greater than the ingestion velocity. Since the ingestion velocity is very high, the ignition time must be very short. Only hydrogen fuel can be used in these scramjet engines because of the very short ignition time. Unfortunately, liquefied hydrogen has a very low density which requires fuel tanks five times larger than conventional fuel tanks (which must be thermally insulated since liquefied hydrogen is a very low temperature cryogenic fluid). This results in a high inert structural mass with a corresponding decrease in payload mass. See "Will the Aerospace Plane Work," Technology Review, January 1987, pp. 42-51 by S. W. Korthals-Altres. This problem is compounded by the fact that since the kinetic energy which the vehicle must develop in order to reach orbital velocity is so high (4.times.10.sup.9 Joules/kg) the amount of hydrogen fuel that must be carried by the vehicle to achieve orbital velocity is about 56% of the total initial vehicle mass, even assuming optimal combustion efficiency. See "From Earth To Orbit In A Single Stage," Aerospace America, August 1987, pp. 32-34, by R. A. Jones and C. D. Donaldson. Thus, there are inherent fundamental engineering problems with this propulsion concept that cannot be circumvented even if the scramjet propulsion system can be made to operate as envisioned at orbital velocity (which may be a physical impossibility). Moreover, another problem will be introduced involving a severe limitation on payload size because nearly all of the interior volume of the vehicle will have to be filled with liquefied hydrogen.
Since the scramjet propelled reusable aerospace plane is currently believed to represent the cheapest method for achieving orbit (with an estimated cost of $200/lb compared to $2,000/lb for the Space Shuttle) the prospects for commercial space travel by private individuals in the 21st century appear to be very remote. (For example, it would cost a 200 lb passenger without luggage $40,000 to be transported to orbit on a one-way flight with the scramjet propelled vehicle.)
By utilizing the concept of beamed energy transmission where a vehicle's propulsive energy is generated off the vehicle at some remote power generating plant and transmitted to it via microwave or laser beams, it is possible to construct a ramjet which does not require the vehicle to carry any fuel. A microwave propelled ramjet was proposed as early as 1959 by William Brown. (See his U.S. Pat. No. 3,083,528 entitled "Microwave Engines" filed May 12, 1959.) This microwave propelled ramjet concept was later developed by Schad and Moriarty to propel payloads from the earth's surface directly into orbit. (See "Microwave Rocket Concept," International Astronautical Congress, Vol. 16, Athens, 1965, pp. 175-199.) Unfortunately, the problem of designing a sufficiently large receiving antenna for capturing the beamed microwave power and focusing it into the ramjet engine required for long range power transmission rendered the concept impractical. Since the wavelength of optical radiation is much smaller than micro wave radiation, it is possible to achieve significantly greater power transmission distances using a laser beam as the energy transmitting medium. A laser propelled ramjet was invented in 1972 by Kantrowitz and Rose. (See U.S. Pat. No. 3,818,700 entitled "Ram Jet Powered By A Laser Beam".) However, all of these prior art beamed energy ramjet propulsion concepts generate propulsive thrust by using the incoming beam energy to heat the ingested air to high temperatures so that it can be expelled through a conventional exhaust system with a velocity greater than the ingestion velocity. Thus, the incoming beam energy is not converted directly into propulsive thrust. These systems require the intermediate step of heating. Since all thermal rocket engines require a relatively small heating region, there is a practical upper limit on the amount of beam power that can be converted into propulsive thrust in thermal rocket engines that is set by power density limitations and by the thermal limitations of the surrounding structure. This intermediate heating step also results in a considerable reduction of efficiency. (More than 50% of the beam energy is wasted by radiative heat losses.) Since laser generators are not very efficient to begin with, prior art beamed power propulsion systems using laser beams will consume large amounts of input energy in the conversion to vehicle kinetic energy.
In theory, the most efficient method for propelling payloads into orbit is by means of an electromagnetic accelerator because the cost essentially reduces to the cost of generating an amount of electrical energy equal to the kinetic and potential energy of the total mass that is accelerated to orbit. For example, if the cost of generating electrical energy is 10.cent./KW-hr, this cost is 90.cent./kg or 41.cent./lb for a 200 km high circular orbit. This is 5,000 times cheaper than the U.S. Space Shuttle and about 500 times cheaper than the proposed aerospace plane. Although there are several different types of prior art electromagnetic accelerators (which are also called mass drivers) that have been designed to accelerate bodies to high velocities (i.e., orbital velocities) from the earth's surface, they all have one common and very undesirable characteristic: they all require an evacuated launch tube through which the payload is accelerated. Therefore, unless an evacuated tube of several hundred kilometers is provided, the acceleration of prior art electromagnetic ground to orbit launchers are inherently high, and the mass and physical dimensions of the payload are too small to be of any significant practical value. See "Electromagnetic Launchers," IEEE Transactions On Magnetics, Vol. MAG-16, No. 5, Sept. 1980, pp. 719-721 by H. Kolm et al.
Large objects, such as completely assembled space based interplanetary transfer vehicles (ITVs) with diameters exceeding 25 m and lengths exceeding 100 m would be completely impossible to accelerate to orbit from the earth's surface by any prior art electromagnetic accelerator. In fact, completely assembled payloads with these dimensions could not be accelerated into orbit by any prior art ground to orbit transportation system (or any such system proposed for the future) because they would simply be too large to fit inside any launch vehicle. It is assumed without question and taken for granted that large objects designed for operating in earth orbit will have to be transported there piece by piece, in relatively small sections, and assembled in orbit.
The high cost and payload limitations of prior art launch vehicles is a result of the basic thrust generating principles used for their propulsion. For all practical purposes, these prior art propulsion principles are believed to be essentially unchangeable because they involve basic laws of physics. However, the discovery of a fundamentally new physical phenomenon or principle be applied to develop a completely new thrust generating propulsion concept. The electromagnetic ramjet propulsion concept disclosed herein is based upon one such discovery--superconducting materials with high critical temperatures. It will be shown that this propulsion concept will enable payloads to be orbited with mass and physical dimensions far beyond that which were previously believed to be possible. Moreover, since this propulsion concept is basically electromagnetic, it also enables payloads to be transported to orbit with minimum cost. The vacuum environment usually required for electromagnetic accelerators is provided by operating the ramjet in the ionosphere at very high altitude--instead of on the earth's surface inside a vacuum tube.